Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through a film of interest. Typical films subjected to permeation testing are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terephthalate (PET), polyvinylidene chloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second cells. The first cell (commonly referenced as the sensing cell) is flushed with an inert gas to remove any target analyte from the cell and the second cell (commonly referenced as the analyte cell) filled with a gas containing a known concentration of the target analyte. A sensor for the target analyte detects the presence of target analyte that has migrated into the sensing cell from the analyte cell through the film.
Permeation instruments typically employ a flow-through method or an accumulation method for sensing the presence of target analyte in the sensing cell. Briefly, the flow-through method uses an inert flushing gas to continuously pick up any target analyte that has migrated into the sensing cell and deliver it to a remote sensor. The accumulation method allows target analyte to build up in the sensing cell for an accumulation period, with the sensor either positioned within the sensing cell or the sensing cell flushed with a flushing gas after the accumulation period for delivery of accumulated target analyte to a remote sensor.
The flow through method allows virtually all sensor types to be used, but are expensive and complex systems. The accumulation method, while permitting the use of less sensitive inexpensive sensors to accurately measure permeation of a target analyte through a film even at very low transmission rates, suffers from significantly longer test times.
Electrochemical sensors are generally preferred for use in permeation instruments as they provide a number of advantages, including (i) extreme accuracy, (ii) ultra-high sensitivity to analyte, (iii) high specificity for a single analyte, (iv) lack of temperature sensitivity, (v) lack of pressure sensitivity, (vi) minimal sensitivity to flow, and (vii) low cost.
Unfortunately, the permeation characteristics of most films are sensitive to humidity and most electrochemical sensors are wet sensors (i.e., the electrolyte is an aqueous solution) that can change the humidity within the sensing cell. Hence, in order to ensure consistent and accurate test results, the humidity within the sensing cell must be controlled.
The humidity within the sensing cell can be controlled for those systems employing the flow through method by controlling the humidity of the inert flushing gas flowing into the sensing cell. Unfortunately, this technique does not work for systems employing the accumulation method as the sensing cell is sealed to fluid flow during testing.
Accordingly, a substantial need exists for a system and method for controlling the humidity within the sensing cell of an analyte permeation testing instrument when the sensing cell is sealed to fluid flow during testing.